An OLED, which is a self-light emitting display, has several advantages of wide viewing angle, rapid response speed, low voltage driving, etc. and has currently been used as a next-generation flat panel display.
A general OLED is basically configured of a multi-layer thin film structure made of organic compounds, such as a hole transporting layer, a light emitting layer, an electron transporting layer, and a hole transporting layer between a cathode and an anode of the OLED, etc. The OLED uses a principle that when electricity is applied between both electrodes, recombines electrons with holes at the light emitting layer by the injection of electrons from the cathode and the injection of holes from the anode and emits light during the drop of energy level from an excited state to a ground state.
Since it has been announced that the OLED using aromatic diamine and Alq3 as materials for forming the light emitting layer is first developed by Eastman Kodak Co. (USA) on 1987 (Appl. Phys. Lett. p 913, (1987)), a study intending to improve performance of naphthyldiamine-based light emitting materials (U.S. Pat. No. 6,549,345), fluorescent-type light emitting materials (U.S. Pat. No. 6,803,121) such as anthracene-based materials, and phosphor-type light emitting materials such as light emitting materials using iridium complex with more excellent efficiency (U.S. Pat. No. 6,858,327) has been progressed. Today, the OLED has actually been applied to a small flat panel display, such as a mobile phone, etc.
As blue light emitting compounds known up to now, there are distryl-based anthrancene derivatives developed by Idemitsu Kosan (EP 388, 768 (1990), Kyushu University (PRO, SRIE, 1910, 180(1993)), or the like. However, the blue light emitting materials have light emitting efficiency lower than light emitting bodies of other colors and should further be improved in thin film stability, heat resistance, etc.
Meanwhile, it is common to use host materials as the phosphor-type light emitting materials so as to maximize efficiency through energy transfer. For example, as low molecular phosphor hosts, there are 4,4′-N,N′-dicarbazole-biphenyl (CBP), 1,3-bis(9-carbazolyl)benzene (mCP), etc (Journal of Materials chemistry (2003) 13, 2157-2163; Journal of Materials chemistry (2005) 15, 2304-2315), as polymeric phosphor hosts, there are poly(N-vinylcarbazole)(PVK), etc. Recently, C. W. Tang group reported that an organic light emitting diode with a single layer structure and a multi-layer structure uses the polymeric polyvinylcarbazole as the phosphor host, instead of using the CBP and the mCP as the phosphor hosts (J. Appl. Phys., Vol. 92, No. 7, 3447). However, the organic light emitting diode has a disadvantage of deterioration of devices by driving heat generated when it is driven for a long time. In particular, since the material, such as the mCP, has low glass transition temp. −Tg, it is easily crystallized into the thin film material when the device is driven, thereby causing a large disadvantage of a change in color purity and a reduction in device lifetime by the driving heat. Therefore, in material development for the long-time and high-efficiency OLED, the heat resistance as an important variable in addition to the color purity and the efficiency has been considered as an important variable.
The inventors have developed materials with excellent light emitting property as well as heat resistance by introducing phosphaphenanthrene derivatives being phosphorous compounds as materials for the OLED from their continued efforts and have successfully performed the OLED configuration using the same.
The phosphaphenanthrene derivatives have been used as only flame retardant, etc. (KR patent publication 1993-3867), but have little example used as materials for the OLED. Polymer materials including phosphaphenanthrene compounds used as the materials for the OLED is known in US Laid-Open No. 2002-0193522 of the related art. As the phosphaphenanthrene compounds included in the polymer materials, the following three compounds are known. However, the known technology is limited to the following three compounds and is not applied to EL.
